1. Field of the Invention
The present invention relates to a method for driving an AC driven plasma display panel (PDP) performing matrix display by a surface electric discharge along a screen.
2. Description of Related Art
The PDP is a flat display device of a self-luminous type, having a pair of substrates as a support. Since a PDP capable of color display was put to practical use, the PDP has wider applications, for example, as a display of television pictures or a monitor of a computer. The PDP is now attracting attention also as a large, flat display device for high-definition TV.
In the PDP using a matrix display system, a memory effect is employed for sustaining a light-emitting state of cells which are display elements. The AC-driven PDP is so constructed to structurally have a memory function by means of a dielectric layer covering electrodes. For displaying an image by the AC-driven PDP, sequential addressing is carried out line by line to select and charge only cells which are to emit light, and then a sustain voltage of alternating polarity for sustaining a light-emitting state, i.e., for sustaining repeated light-emission discharges for display, is applied to all cells simultaneously. The sustain voltage is a predetermined voltage which is lower than a firing voltage, i.e., a discharge start voltage. In a cell having wall charge, the wall charge is superposed on the sustain voltage to form an effective voltage which is actually applied to the cell. When the effective voltage exceeds the firing voltage, an electric discharge takes place and the cell emits light. If the sustain voltage is repeatedly applied at a short cycle, apparently continuous light emission can be obtained. Luminance of display depends on "integrated luminescence intensity" which is the total amount of light emitted during a sustain period for sustaining the light-emission discharges. Usually, the frequency of a sustain voltage pulse which determines a discharge cycle is constant. Therefore the length of the sustain period, i.e., the number of discharges, s set depending on an intended luminance.
As color display devices, AC-driven PDPs of a surface discharge type have become commercial. The surface discharge type is a system wherein pairs of main electrodes, i.e., pairs of first and second electrodes, which alternately become positive or negative for sustaining the light-emission discharges, are arranged in parallel on one of a pair of substrates. Since the main electrodes extend in the same direction, third electrodes intersecting the main electrodes need to be provided for selecting individual cells. The third electrodes are disposed on the other substrate in an opposing relation to the main electrodes with a discharge gas space therebetween in order to reduce electrostatic capacity of the cells. An electric discharge is generated for addressing across one of the main electrodes and the third electrode. In such PDPs having a three-electrode structure, fluorescent layers for color display can be provided on the other substrate opposite to the substrate on which the main electrodes are placed, in order to reduce deterioration of the fluorescent layers by ion impact at electric discharges and to increase the life of the devices. Such a display panel having the fluorescent layers on a rear substrate is called a "reflection type" PDP. On the other hand, a display panel having the fluorescent layers on a front substrate is called a "transmission type" PDP. The reflection type, in which front surfaces of the fluorescent layers emit light, is more excellent in luminous efficiency.
For displaying images, i.e., frames, in time sequence by the AC-driven PDP in which the light-emission discharge is maintained using wall charge as described above, initialization (reset) is carried out to make the entire screen non-charged, during a time period from the end of sustaining the light-emission discharges for a certain image to the beginning of the addressing for the following image. Methods for this initialization fall roughly into two categories: One is to generate a surface discharge regardless of the presence of wall charge. For example, a reset pulse whose crest value is higher enough than a surface discharge start voltage is applied simultaneously to the main electrodes of all cells. A strong discharge occurs at the building-up of the reset pulse, and more wall charge is generated than is generated at the sustaining of the light-emission discharges. Accordingly, the wall voltage cancels the applied voltage and the effective voltage decreases. When the reset pulse falls, the wall voltage remains as the effective voltage, and a self discharge occurs to eliminate the wall charge. The other one is to generate a surface discharge only in cells having wall charge, i.e., cells having been activated to emit light for display in the immediately preceding display.
For the former, conventionally, the initialization is carried out by applying to the main electrodes of all cells simultaneously, a voltage pulse whose pulse width is shorter than that of the voltage for sustaining the light-emission discharges, a voltage pulse whose crest value is a little lower than that of the sustain voltage or a voltage pulse whose voltage gradually rises. In the case where the pulse width is short, new charge by electrostatic attraction is not generated after the wall charge is eliminated by the surface discharge. In the case where the crest value is low, the surface discharge is weak and the new charge is insignificant. In the case where the voltage rises gradually, a relatively weak discharge occurs when the voltage reaches the surface discharge start voltage.
However, where the surface discharge is generated in all the cells for the initialization, the contrast of display drops because the entire screen emits light every time when display is changed. Especially, in the case where the frame is divided for performing gradation display, the contrast falls remarkably since a plurality of changes of display are carried out in one frame.
On the other hand, where the surface discharge is generated only in the cells having wall charge, a particular voltage is applied to the main electrodes for generating the surface discharge. Therefore, if relatively large wall charge is present near the third electrodes, a complete initialization cannot be obtained and there remains difference in discharge probability after the initialization between the cells having been activated in the immediately preceding display and the other cells. For these reasons, a voltage margin of addressing is narrow and stable display can hardly be realized.